The Great Barrier Reef is the world's largest coral-reef systemit's 150 miles longer than the California coastand the most diverse. Every location here has an upside, from spectacular wrecks in the south to hammerhead and minke whale action farther offshore. But the best variety is found several hours off Cairns, in the northern section. Veteran photographer David Doubilet likes Cod Hole, at Ribbon Reef No. 10, where the potato cod are "the size of Newfoundland dogs." Nearby is Pixie Pinnacle, full of lionfish, sea anemones, and sea fans. Hit both spots and more on a three-day, three-night trip out of Cairns on the 120-foot liveaboard Spirit of Freedom (US$1,300 per person, including return flight from Lizard Island; spiritoffreedom.com.au). With its strong tides, the Reef isn't for newbie divers. But go between September and early November, when the seas are calmer on the northern end of the reef, and it will be easier to frame shots of those massive cod. Gear up with the Sea & Sea DX-2G camera-and-housing system ($1,050; seaandsea.com), plus a strobe.